Optical semiconductor parts such as light-emitting diodes (LED) have many advantages including small size, efficiency, vivid color emission, elimination of bulb failure, excellent drive characteristics, resistance to vibration, and resistance to repeated turn-on and off. These parts are often utilized as various indicators and light sources. Optoelectronic devices using optical semiconductor parts are enclosed in cases or packages, which are now typically made of polyphthalamide (PPA) resins.
The current rapid advance of the photo-semiconductor technology has brought about photo-semiconductor devices of increased output and shorter wavelength. Photo-semiconductor devices are often encapsulated or encased using prior art PPA resins as colorless or white material. However, these encapsulants and cases are substantially degraded during long-term service and susceptible to visible color variations, separation and a lowering of mechanical strength. It is desired to overcome these problems effectively.
More particularly, JP 2,656,336 discloses that an optoelectronic device is encapsulated with a B-staged epoxy resin composition, in the cured state, comprising an epoxy resin, a curing agent, and a cure promoter, the components being uniformly mixed on a molecular level. As to the epoxy resin, it is described that bisphenol A epoxy resins or bisphenol F epoxy resins are mainly used although triglycidyl isocyanate and the like may also be used. In examples, a minor amount of triglycidyl isocyanate is added to the bisphenol A or F epoxy resin. The present inventors have empirically found that this B-staged epoxy resin composition for semiconductor encapsulation tends to yellow when held at high temperatures for a long period of time.
Triazine derived epoxy resins are used in LED-encapsulating epoxy resin compositions as disclosed in JP-A 2000-196151, JP-A 2003-224305, and JP-A 2005-306952. None of these patents succeed in solving the problem of yellowing during long-term service at high temperature.
JP-A 2006-77234 describes a LED-encapsulating resin composition comprising an organopolysiloxane having a weight average molecular weight of at least 5×103 and a condensation catalyst. Since this organopolysiloxane must be transparent and liquid at room temperature, the composition does not lend itself to transfer molding and compression molding.
Reference should also be made to Japanese Patent No. 3,512,732, JP-A 9-310007, JP-A 2001-234032, JP-A 2001-316591, JP-A 2002-302533, and Electronics Packaging Technology, April 2004.